1. Technical Field
The present invention relates to a radar apparatus which carries out a transmission and reception of an electric wave using a plurality of antennas.
2. Related Art
In general, in a radar apparatus, an angle of a target can be obtained by scanning an antenna beam narrowed down to a desired width. Scanning methods can be divided largely into mechanical scanning methods and electronic scanning methods and, as one electronic scanning method, there is a digital beam forming (hereafter abbreviated as a DBF).
The DBF is a method in which an electric wave transmitted from a transmitting antenna and reflected off the target is received by a plurality of receiving antennas at one time, and various antenna patterns are formed by a digital signal processing, using the received signals. It is conceivable that, in a heretofore known phased array type antenna, a function of an analog phase shifter equipped on each antenna, and a function of analogically synthesizing outputs from the analog phase shifter, are realized, in the DBF, by the digital signal processing.
According to the DBF, as it is not necessary to drive the antennas as in the mechanical scanning method, there is no need for a drive mechanism and, for this reason, it has features of being resistant to vibration, and being able to achieve a reduction in size and weight. Also, in comparison with the phased array type antenna, as there is no need for the analog phase shifter, it has a feature of being able to achieve a reduction in costs.
In general, it has been known that, in a case of obtaining the angle of the target by means of the DBF, the larger an antenna aperture diameter, the superior an angular resolution. However, the angular resolution has a limitation under a constraint such as disposing the radar apparatus in a limited space. Therein, JP-A-2004-198312 discloses a method which, by providing two transmitting antennas, one at either end of the plurality of receiving antennas, transmitting electric waves from the two transmitting antennas in time divisions, and implementing the DBF using each reception result, realizes an equivalent aperture approximately twice as large, and enhances the angular resolution.
A description will be given, using FIG. 11, regarding a fact that it is possible, by transmitting the electric waves from the two transmitting antennas in time divisions, to equivalently increase the aperture. In FIG. 11, a left half of the figure shows, by dotted lines, equal phase planes of an electric wave incident on each receiving antenna R1 to R4 at a transmitting antenna T2 transmission time, and a right half, at a transmitting antenna T1 transmission time. Herein, a case is assumed in which the received waves return from a direction of θ on a right side from a front direction. A transmission/reception path difference of electric waves incident on the receiving antennas R1 to R4, with T2 as a reference at the transmitting antenna T2 transmission time, is as follows.
R1+4ΔrR2+3ΔrR3+2ΔrR4 +ΔrHerein, in the event that a receiving antenna spacing is d, the path difference is expressed byΔr=d sin θ
Meanwhile, a transmission/reception path difference of electric waves incident on the receiving antennas R1 to R4 with T2 as a reference at the transmitting antenna T1 transmission time, as the transmission path difference is +5Δr, is as follows.
R1+9ΔrR2+8ΔrR3+7ΔrR4+6Δr
Therefore, a transmission/reception phase difference with the transmitting antenna T2 as the reference can be expressed as follows.
At the T2 transmission time,
R1−4ΔφR2−3ΔφR3−2ΔφR4 −ΔφAt the T1 transmission time,
R1−9ΔφR2−8ΔφR3−7ΔφR4−6Δφ
Herein, regarding a virtual alignment of a position of the transmitting antenna T1 with T2, it is sufficient that the transmission/reception phase difference in the receiving antennas R1 to R4 at the transmitting antenna T1 transmission time is made +10Δφ (a phase correction).
At this time, the phase difference being as follows at the T1 transmission time,
R1 +ΔφR2+2ΔφR3+3ΔφR4+4Δφit is possible to virtually treat a receiving antenna disposition in such a way that a total of eight receiving antennas are disposed sandwiching the transmitting antennas T2/T1. That is, although there actually are only four receiving antennas, it is possible to virtually realize approximately twice the antenna aperture diameter, and to enhance the angular resolution.
In the DBF, in order to form beams in a desired orientation direction, it is necessary to phase correct each received signal obtained by the plurality of receiving antennas but, as a common technique thereof, there is a discrete Fourier transform (hereafter abbreviated as a DFT). In a case of forming beams in a plurality of orientation directions, it follows that the DFT is repeatedly executed assuming one direction, and then another direction, meaning that a calculation amount increases in the event that there are many directions to be assumed.
Meanwhile, as a fast Fourier transform (hereafter abbreviated as an FFT) is an algorithm which can carry out the DFT at a high speed, from a viewpoint of the calculation amount, it is desirable to use the FFT in the DBF. This is because, in the event that a signal length is N, the calculation amount of the FFT is proportional to N log N, while a Fourier transform which is not speeded up is proportional to N2.
In a case of providing the two transmitting antennas, one at either end of the plurality of receiving antennas, such as one shown in JP-A-2004-198312, as shown in FIG. 12, an equivalent receiving antenna disposition becomes a kind of condition in which an empty space equivalent to twice a transmitting and receiving antenna spacing D is formed between CH3 and CH4. Herein, in order to differentiate signal outputs corresponding to the receiving antennas R1 to R4 at the transmitting antenna T1 transmission time from signal outputs corresponding to the receiving antennas R1 to R4 at the transmitting antenna T2 transmission time, channels (CH's) are defined as follows.
At the transmitting antenna T2 transmission time,
R1CH0R2CH1R3CH2R4CH3At the transmitting antenna T1 transmission time,
R1CH4R2CH5R3CH6R4CH7
In a case of a receiving antenna disposition such as one shown in FIG. 12, in the event of intending to implement the DBF using the FFT, a form is taken in which virtual CH's are disposed between CH3 and CH4 at the same spacings as the receiving antenna spacing d, and the virtual CH's are zerofilled. However, as a CH disposition including the virtual CH's becomes unequally spaced depending on how the transmitting and receiving antenna spacing D is done, it is impossible to apply the FFT, and it is necessary to implement the DFT. That is, there is a problem of causing an increase of the calculation amount.